This invention relates to a device including a portion of phosphate glass which can be controllably dissolved to perform a function, for example to release controlled quantities of an active material into an aqueous environment at or after a pre-determined time interval. The invention also relates to a method for controlling the dissolution of a phosphate glass.
There is a need for the release of active materials such as drugs or pesticides into an aqueous environment at or after a pre-determined time after being placed in contact with the aqueous environment, or continuously at a predetermined rate. It is known to employ glasses which act as a barrier between the aqueous environment and the active material and which dissolve in the aqueous environment over a period of time thereby to release the active material at or after a pre-determined time Period. It is also known to employ such glasses which themselves incorporate the active material. Previous work in this field includes the work reported in UK-B-2057420 which dealt with the overall control of the dissolution of glasses and exemplified phosphate glasses with a P.sub.2 O.sub.5 content in excess of 50 mole %.
The glasses disclosed in UK-B-2057420 suffer from the disadvantage that controlled dissolution of the glasses is not possible since, for glass compositions having greater than 50 mole % P.sub.2 O.sub.5, the solution products are hindered from leaving the glass surface because there is a tendency for gelatinous films to form adjacent to the glass surface. The gelatinous films interfere with the dissolution of the glass and render the dissolution rate time - dependent i.e. the dissolution rate decreases with time. This means that the dissolution of the glass cannot be reliably controlled.